Photoconductive layers have long been employed as a means for obtaining from a limited light exposure an image-wise pattern of conductivity which can be utilized in a number of electrolytic processes for generating visible, highcontrast, substantially permanent graphic images. Photoconductive layers have thus been employed, for example, in imaging methods which entail the electrolytic reaction of color formers to provide a visible image, as described in U.S. Pat. No. 2,764,693. As a means for providing the definitive image pattern of electrical conductivity, photoconductive layers have also been employed in electropolymerization methods as described in U.S. Pat. No. 3,600,173.
The present invention also involves electropolymerization of ethylenically unsaturated compounds, such as vinyl monomers, and in this respect is sufficiently closely related to the subject matter of the latter referenced patent that the disclosures and discussions there, particularly with respect to polymerizable monomers and photoconductive materials per se, will provide a significant and substantial description of the basis of the present invention.
The invention described in U.S. Pat. No. 3,600,173, while providing a useful means for obtaining images comprising polymerized vinyl materials, does have some notable drawbacks. A first and not the least of these disadvantages is the fact that the progenitor of polymerization-initiating free radicals present in the imaging material consists essentially of a light-sensitive diazonium compound. As a result of the inherent potential in the imaging material for decomposition of this essential element of the electropolymerization process, such materials must generally be handled under conditions which will ensure the minimum of degeneration of the free radical progenitor. For example, the materials must be stored in temperate conditions with the exclusion of light and, during use, safelight procedures must be observed.
An additional drawback, and one of greater significance, results from the fact that the electropolymerization procedures utilizing diazonium free radical progenitors involves a reaction which is cathodic by nature, as described in the referenced disclosure. This characteristic coupled with the inherent rectification properties of generally preferred photoconductor materials -- zinc oxide, for example -- results in the formation of polymer images at the interface between the photoconductor and the polymerizable composition of the imaging material.
While such a disposition of products of electrolysis at the photoconductor surface is not detrimental, and may even be preferable in the direct-imaging systems earlier noted and those additionally described, for example, in U.S. Pat. No. 3,010,883, the polymerization of monomeric ingredients at the surface farthest disposed from the carrier sheet of an imaging composition is far from advantageous. This result derives from the fact that polymerizable imaging compositions are employed, for the most part, in wash-off procedures where the polymerized matrix must be closely adhered to the support of the material while the unexposed and, thus, non-polymerized materials are removed from the carrier sheet by washing or the like.